The invention relates to a process for the manufacture of mouldings, especially optical lenses and specifically contact lenses, to a corresponding device for the manufacture of mouldings, and to mouldings, especially optical lenses and specifically contact lenses, manufactured or obtainable in accordance with that process or by means of that device, in accordance with the preamble of the respective independent patent claim.
Contact lenses that are to be manufactured economically in large numbers are preferably produced by the so-called mould or full-mould processes. In those processes, the lenses are manufactured in their final form between two moulds, so that neither subsequent machining of the surfaces of the lenses nor machining of the rim is necessary. Mould processes are described, for example, in PCT Patent Application publication No. WO 87/04390 and in European Patent Application publication No. 0 367 513.
In those known mould processes, the geometry of the contact lens that is to be produced is determined by the mould cavity. The rim of the contact lens is also formed by the mould, which usually consists of two mould halves. The geometry of the rim is determined by the contour of the two mould halves in the region in which they are in contact with one other.
In order to produce a contact lens, first of all a determined amount of the flowable starting material is introduced into the female mould half. The mould is then closed by applying the male mould half. Normally, the starting material is supplied in slight excess, so that the excess amount is forced into an overflow chamber externally adjacent to the mould cavity when the mould is closed. The subsequent polymerisation or crosslinking of the starting material is effected by irradiation with UV light or by thermal action or by another, non-thermal, method, during the course of which both the starting material in the mould cavity and the excess material in the overflow chamber are fully cured. The full cure of the excess material may be delayed slightly, since it may initially be inhibited by atmospheric oxygen. In order to achieve fault-free separation of the contact lens from the excess material, the excess material must be well sealed off or expelled from the zone in which the two mould halves are in contact with one another. Only in that manner is it possible to obtain fault-free contact lens rims.
The materials currently used for the moulds are preferably plastics, such as, for example, polypropylene. The moulds are produced by injection moulding and used only once (disposable moulds). The reason for this, inter alia is that the moulds are in some cases contaminated by the excess material, are damaged when the contact lens is separated, or are irreversibly deformed in some areas.
In the case of injection-moulded moulds, variations in the dimensions must also be expected, as a result of variations in the manufacturing process (temperatures, pressures, material properties). Shrinkage of the moulds may also occur after injection moulding. The dimensional variations in the mould may lead to variations in the parameters of the contact lens being produced (vertex refractive power, diameter, base curve, middle thickness etc.), which may have an adverse effect on the quality of the lenses and thus result in a reduced yield. If the seal between the two mould halves is inadequate, the excess material is not cleanly separated, which may result in the formation of so-called flash at the contact lens rim. Where that is relatively pronounced, such a cosmetic fault at the rim of the lens may also cause irritation to the wearer, and such lenses therefore have to be identified by inspection and removed.
Particularly in view of the quality requirements of the contact lens rim, the moulds air also used only once, because it is not possible to rule out absolutely a certain deformation of the moulds in the region in which they are in contact with one another.
A further mould process for the manufacture of, inter alia, contact lenses is described in U.S. Pat. No. 4,113,224. That process uses a mould in which the cavity is not completely sealed but is connected by a thin annular gap to an annular reservoir channel (overflow channel) surrounding the cavity. During the crosslinking process, material can flow back from the reservoir through the annular gap and into the mould cavity to compensate for the relatively large shrinkage in volume which occurs with the lens materials customarily used.
The material in the reservoir channel can be prevented from crosslinking by an inhibiting gas atmosphere or by being shielded from the energy radiation causing the crosslinking. To ensure that material flows back into the mould cavity, the material located in the mould cavity is, at least to begin with, subjected to radiation only in a central region, which is smaller than the diameter of the mould cavity, or is exposed to a stronger intensity of radiation in that central region than in the edge region of the mould cavity surrounding that central region. After crosslinking has commenced in the central region and has progressed to a certain degree, the edge region too, however, together with the adjacent annular gap and the material located in the reservoir channel, is exposed to the full radiation and crosslinked. The burrs and flash mentioned above are inevitably formed, so that contact lenses and other mouldings manufactured by that known process require subsequent mechanical processing.
The aim of the present invention is to develop further and so improve a process and a device of the generic types that the difficulties and problems described above in the example of the manufacture of contact lenses are avoided In particular, conditions are to be created to allow re-use of the necessary moulds or mould halves and avoid the formation of burrs and flash on the manufactured mouldings, so that the mouldings will have an extremely low reject rate and mechanical or other subsequent processing of the mouldings will be dispensed with.
The aim underlying the invention is achieved by the measures and features described in the latter part of the independent process claim and of the independent device claim. Especially expedient and advantageous arrangements and further developments of the process of the invention and of the device of the invention are given in the dependent claims.
xe2x80x9cCrosslinkingxe2x80x9d, here and in the following, is to be understood as meaning, briefly, any kind of reaction in which the material is converted by polymerisation of a suitable monomer, oligomer and/or prepolymer and/or a mixture thereof into a state in which it retains the shape defined by the mould cavity. Suitable materials and polymerisation/crosslinking reactions are known to the person skilled in the art and typical examples are to be found, inter alia, in the mentioned U.S. Pat. No. 4,113,224 and the publications mentioned therein.
According to the general underlying concept of the invention, therefore, the polymerisation or crosslinking of the starting material is restricted solely to the region of the moulding, specifically the contact lens, being produced. Any excess material present is not polymerised or crosslinked. In the process according to the invention partial areas of the moulding rim are formed not by a mechanical limitation of the material by mould walls but by a spatial limitation of the impinging energy (usually UV or some other radiation) that triggers the polymerisation or crosslinking. As a result of those two measures, contact between the two mould halves can in a preferred arrangement be avoided, so that they are not deformed and can accordingly be used again. In addition, the known problem of volume shrinkage which occurs during crosslinking can also be dealt with very simply thereby without it being necessary, as in the case, for example, of U.S. Pat. No. 4,113,224, for the moulding to be mechanically processed subsequently.